Molecular-sieve catalysts are commonly utilized to upgrade petroleum fractions and to produce a variety of petrochemical products. Reactions such as cracking, hydrocracking, reforming, isomerization of aromatics and aliphatics, polymerization, alkylation, dealkylation, transalkylation and disproportionation are effectively and efficiently promoted by catalysts featuring molecular sieves. The most widely used of the molecular sieves are the crystalline aluminosilicate zeolites formed from corner-sharing AlO.sub.2 and SiC.sub.2 tetrahedra. The zeolites generally feature pore openings of uniform dimensions, significant ion-exchange capacity and the capability of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure. Catalytic properties of zeolites, e.g., acidic activity and shape selectivity, are controlled by such parameters as selection of zeolite type and pore dimensions, crystal size, silica/alumina ratio, and the nature of cations used to balance the electrovalence of tetrahedral aluminum.
Methods have been disclosed for modifying the catalytic activity of zeolites in defined process uses. U.S. Pat. No. 4,508,836 (Haag et al.) teaches a process for converting an aromatic-containing feedstock using a crystalline aluminosilicate zeolite which has been precoked. Oligomerization of olefins using a crystalline silicate which has been surface-inactivated by steaming and coking is disclosed in U.S. Pat. No. 5,234,875 (Han et al.). Patent application Ser. No. 997,831 discloses high-severity followed by moderate-severity C.sub.8 -aromatics isomerization.
More recently, a class of useful non-zeolitic molecular sieves containing framework tetrahedral units (TO.sub.2) of aluminum (AlO2), phosphorus (PO.sub.2) and at least one additional element EL (ELO.sub.2) has been disclosed. "Non-zeolitic molecular sieves" or "NZMS" include the "SAPO" molecular sieves of U.S. Pat. No. 4,440,871 (Lok et al.), "ELAPSO" molecular sieves as disclosed in U.S. Pat. No. 4,793,984 (Lok et al.), "MgAPSO" sieves of U.S. Pat. No. 4,758,419 (Lok et al.) and crystalline metal aluminophosphates--MeAPOs where "Me" is at least one of Mg, Mn, Co and Zn--as disclosed in U.S. Pat. No. 4,567,029 (Wilson et al.). Framework As, Be, B, Cr, Fe, Ga, Ge, Li, Ti or V and binary metal aluminophosphates are disclosed in various species patents. U.S. Pat. No. 4,861,938 discloses light-olefin production over a NZMS catalyst with catalyst deactivation, at least partial regeneration and further conditioning of the catalyst with a basic material.
The use of an NZMS-containing catalyst for the isomerization of butenes is disclosed in U.S. Pat. No. 5,132,484 (Gajda). U.S. Pat. No. 5,107,050 (Gaffney et al.), discloses butene isomerization using a MgAPSO or SAPO molecular sieve at a temperature above 900.degree. F.
Non-zeolitic molecular sieves as used in catalysts have unique characteristics of pore dimension, acidity, etc. which affect activity and selectivity in certain hydrocarbon-conversion applications. One problem facing workers in the art is how to moderate and control activity over the life of the catalyst to obtain favorable results.